The present invention relates to an alignment technique and more particularly to a technique which is effective in the application to alignment of a semiconductor wafer (hereinafter referred to simply as "wafer") with respect to a mask in a reduced projection exposure step in the production of a semiconductor device.
For transferring a circuit pattern formed using a light shielding film on a mask such as a reticle onto a wafer, there usually is adopted a method which employs a reduced projection exposure system. In this case, the wafer-mask alignment is performed in the following manner for example.
Illumination light is radiated through a reduced projection lens to an alignment mark formed in a stepped uneven shape on the surface of a wafer, then reflected light from the alignment mark is made incident on a TV camera through a beam splitter, etc., and an electric signal is detected on the basis of the quantity of light of the reflected light to thereby grasp the position of the alignment mark and effect positioning of an area to be exposed of the wafer relative to the mask. Such a positioning technique is generally called "through the lens" (TTL) method.
As a literature pointing out problems to be solved of the TTL method there is "Nikkei Micro Devices," Nikkei McGraw-Hill Inc. (Dec. 1, 1987), pp.70-72.
An alignment technique commonly adopted according to the TTL method will be explained below with reference to FIG. 5.
In FIG. 5, the numeral 71 denotes a wafer as an object to be exposed; numeral 72 denotes a reduced projection lens for exposure which is positioned just above the wafer 71; numeral 73 denotes a reticle as a master plate; numeral 74 denotes a TV camera as a recognizer; and numeral 75 denotes a mercury lamp serving as both an exposure light source and an illumination light source. On the optical path of the reduced projection lens 72 and TV camera 74 there are disposed a reflecting mirror 76, a relay lens 77 and a beam splitter 78. Light transmitted by the beam splitter 78 is received by the TV camera. On the other hand, between the mercury lamp 75 and the beam splitter 78 are disposed a band pass filter 80 which permits only E-line (546 nm) to pass therethrough out of rays provided from the mercury lamp 75, and a condenser lens 81.
Thus, E ray, which is a monochromatic light ray, is used as an illumination light for the detection of a pattern, while at the time of exposure there is used G-line (436 nm) which is an exposure light.
The illumination light is radiated by the beam splitter 78 onto the wafer 71 via the relay lens 77, reflecting mirror 76 and reduced projection lens 72, and the reflected light travels backward through the above path and is incident on the TV camera 74. On the basis of the image recognized by the TV camera 74 there is made detection of waveform and a central position of such an alignment mark 6 on the wafer as shown in FIG. 4(a) is calculated.
Further, in FIG. 17(a), numeral 201 denotes a wafer as an object to be exposed; numeral 202 denotes a reduced projection lens for exposure disposed just above the wafer 201; numeral 203 denotes a reticle as a master plate; numeral 204 denotes a TV camera as a recognizer; and numeral 205 denotes a mercury lamp as an exposure light source. On the optical path of the reduced projection lens 202 and TV camera 204 are are disposed a reflecting mirror 206, a relay lens 207 and a half mirror 208. Reflected light transmitted through the half mirror 208 is received by the TV camera 204. On the other hand, between the mercury lamp 205 and the half mirror 208 are disposed a band pass filter 210 which permits only E-line (546 nm) to pass therethrough out of the rays provided from the mercury lamp 205, and a condenser lens 211.
Illumination light emitted from the mercury lamp 205 is applied onto the wafer 201 via the half mirror 208, relay lens 207, reflecting mirror 206 and reduced projection lens 202, while the reflected light from the wafer 201 travels backward along the above optical path and is received by the TV camera 204, and on the basis of the image recognized by the TV camera 204 there is made waveform detection.
FIG. 17(b) is a partially sectional view schematically showing an alignment pattern 212 formed on the wafer 201.
The alignment pattern 212, which is formed in synchronism with a wiring pattern, etc. on the wafer 201, has a concave section with both inner side-ends formed edge-like. Illumination light is radiated vertically from above to the wafer surface formed with such alignment pattern 212, and it has been premised that the light reflected by the wafer surface will travel backward along the same optical path as that of the illumination light.
As shown in the same figure, when the alignment pattern 212 is formed on the wafer 201 in an ideal condition without distortion, the resulting waveform exhibits peak values at the paired edge portions as shown in FIG. 17(c). A central position [(X.sub.R +X.sub.L)/2] of the alignment mark 212 is determined by calculation on the basis of coordinates X.sub.R, X.sub.L of both peak values, and alignment is effected using it as a reference value X.sub.O.
Further, a brief explanation will be made below about prior art publications which may be related to the present invention.
In Suzuki et al's Japanese Patent Laid-Open No. 293718/87 (laid open Dec. 21, 1987) there is disclosed a correction lens system for the correction of spherical aberration, astigmatism and coma aberration in a stepper having aligning optical system using a single wave length light.
In Komoriya et al' Japanese Patent Laid-Open No. 177625/85 (laid open Sep. 11, 1985) there is shown an example of inserting a chromatic aberration correcting lens system in an off-axis portion of an on-axis aligning optical system in a stepper having an aligning optical system using a continuous spectrum.
In Sugiyama's Japanese Patent Laid-Open No. 203640/86 (laid open Sep. 9, 1986) there is disclosed a struture for conducting light which has passed through a reticle or a mask to a detection system through a chromatic aberration correcting lens in an aligning optical system of a stepper similar to the above.
Further, in Shiba et al's Japanese Patent Laid-Open No. 251858/86 (laid open Nov. 8, 1986) it is disclosed that a cylindrical inner-surface mirror is used in the feed path of an exposure light source.